1. Field of the Invention
The present invention relates to a delay line device used to delay an electric signal such as a pulse signal employed in an electronic apparatus such as a communication device and a computer, and to a method of manufacturing such delay line devices.
2. Description of the Prior Art
As one of such delay line devices, there has heretofore been known a non-distributed constant type delay line device wherein a coil and a capacitor have been combined together. Since, however, the upper limit of a frequency to be used is low, this delay line device cannot be applied to a high-speed transmission circuit.
In order to improve this frequency characteristic, there has been proposed a distributed constant type delay line device disclosed, for example, in U.S. Pat. No. 4,641,113. This delay line device can handle a high-speed signal (i.e., a high-frequency signal) having a pulse width or rise time of 1 nanosecond or less.
This type of delay line device is of a microstrip line structure. As is apparent from FIG. 14 showing a schematic cross section of the delay line device, the delay line device has a signal conductor or line 4 bent in a zigzag manner, for example, which is formed on one of main surfaces of a dielectric substrate 2, and a ground electrode 6 formed on substantially the entire area of the other main surface thereof.
Since, however, the signal line 4 is electrically exposed, the delay line device has a problem that the signal line 4 is subjected to electromagnetic influences from the outside and electromagnetic noise is also produced from the signal line 4 itself.
Such a problem can be solved by making use of a delay line device of a so-called triplate type stripline structure provided with dielectric substrates 2 and ground electrodes 6 respectively formed on the upper and lower surfaces of a signal line 4 as shown in FIG. 15 representative of a schematic cross section of the delay line device. Such a triplate type delay line device is, however, accompanied by a problem that the accuracy of geometric dimensions between the signal line 4 and the respective upper and lower ground electrodes 6 exerts a substantial influence upon electrical characteristics such as an impedance and a delay time (or a time delay).
Described more specifically, it is necessary to set distances L.sub.1 and L.sub.2 between the signal line 4 and the upper ground electrode 6 and between the signal line 4 and the lower ground electrode 6 so as to be equal to each other and to uniformly set the distances over the entire length of the signal line 4. If these distances L.sub.1 and L.sub.2 vary, the impedance of a signal transmission line varies. As a result, an increase in reflection of a signal is made and waveform distortion is developed. A variation in impedance exerts an influence even on a delay time. The impedance and the delay time greatly vary according to the frequency, particularly in a high-frequency range.
Upon actually fabricating the aforementioned triplate type delay line device, resinous substrates are conventionally used as the dielectric substrates 2 to bond the upper and lower dielectric substrates to each other with the signal line 4 interposed therebetween. Then, the resinous substrates are bonded to each other by heating under pressure. Alternatively, where ceramic substrates are used as the dielectric substrates 2, a means such as a method of interposing a bonding resinous sheet between the ceramic substrates is used. In either case, the geometric dimensions between the signal line 4 and the upper and lower ground electrodes 6 vary subtly. It was, therefore, difficult to materialize a delay line device whose electrical characteristics remain stable up to a high-frequency range.
On the other hand, a size reduction in recent electric parts and the integration of the same into a high density, which have been required of the electric parts, are similarly required of the delay line device. There is now a demand for the development of a delay line device which is small in size, long in delay time and can respond to a high-speed signal. In order to meet such a demand, such a delay line device as has been disclosed in Japanese Utility Model Laid-open Gazette No. 2-92210 was developed. This type of delay line device is constructed in the following manner. As illustrated in FIG. 16 in an exploded manner, the aforementioned signal lines 4 are formed on respective ones of main surfaces of dielectric substrates 2 and ground electrodes 6 are formed on substantially the entire regions of the other main surfaces of the dielectric substrates 2. The products, i.e., dielectric substrates thus obtained in plural form are bonded to one another by heating under pressure with sheets 8 made of organic materials interposed therebetween. The so-produced delay line device has a multi-layered triplate type stripline structure. The signal lines 4 provided within the respective layers are electrically series-connected to one another via through holes or the like.
This type of delay line device is small in size, long in delay time and can cope with a high-speed signal. It is also possible to solve the aforementioned problem of electromagnetic induction since the delay line device is of the triplate type. However, the geometrical dimensions between the signal lines 4 and their corresponding upper and lower ground electrodes 6 vary subtly since the sheets 8 are used to bond the respective dielectric substrates 2 formed with the signal lines 4 to one another. Thus, a problem still remains that difficulty rises in ensuring the stability of electrical characteristics up to a high-frequency range.